The technology combining micromachining and integrated circuit processes becomes mature. Thus, for many micro-electro-mechanical system (MEMS) devices, microactuators are used to provide driving force and play a key part in operation. So far, electromagnetic microactuators are the main stream because the making processes are quite simple. In general, the electromagnetic microactuators are composed of a magnet and a planar coil. The planar coil can be obtained by integrated circuit processes.
The planar coil is applied to many MEMS devices. A planar coil can be used with a permanent magnet to drive micro pumps, micro valves, micro relays or other elements. Manufacture of planar coils is very important in such a field.
Please refer to FIGS. 1(A) to 1(E). A conventional method for making a planar coil is shown. A substrate 1 is provided, as shown in FIG. 1(A). Then a number of trenches 2 are formed by reactive-ion etching, as shown in FIG. 1(B). The trench 2 has a trapezoidal cross section. A barrier 3 is formed on the substrate 1. Later, a seed layer 4 is formed on the barrier 3, as shown in FIG. 1(C). Next, copper is electroplated onto the seed layer 4 until the trench 2 is filled with copper, as shown in FIG. 1(D). Finally, the seed layer 4 on the surface of the substrate 1 is removed. Accordingly, a planar coil is formed.
An obvious defect is that the cross section of the coil is trapezoidal. Usually, gaps are formed in the coil during a cryogenic process or Bosch process while making high aspect ratios of cross section for the planar coil, causing poor electrical properties of the coil. For a planar coil having a trapezoidal cross section, aspect ratio is limited.
Please refer to FIG. 2 and FIG. 3. Another conventional method for making a planar coil is shown. A tooling 11 using a steel-made plate 10 is engraved with many peaks 14 and troughs 15. A hardening layer 13 is formed on a polymer substrate 12 by ultraviolet beams. Then, the hardening layer 13 is pressed by the tooling 11. Due to the peaks 14 and troughs 15, a number of high aspect ratio trenches will be formed on the hardening layer 13. Next, a seed layer 16 of copper is formed by vapor deposition process on the hardening layer 13. A coil layer 17 is electroplated onto the seed layer 16. Finally, after the redundant portion of the coil layer 17 is removed, a planar coil is formed.
The method utilizes the tooling 11 and the hardening layer 13 to provide high aspect ratio trenches for making the planar coil. No matter what additional agent is used to enhance the hardness of tooling 11, the tooling 11 will be worn out after long-term use. A designated shape of the planar coil can not be achieved. In addition, the coil layer 17 will stick to the hardening layer 13 if the hardening layer 13 is not firm enough.
From the description above, a method for making planar coils with good quality is desired. Especially, a method for making planar coils with high aspect ratio is desperately needed.